fix: use shared event + .athrow() to properly set stream_was_cancelle… (#9019)

fix: use shared event + .athrow() to properly set stream_was_cancelled flag

**Problem:**
When a run is cancelled via /cancel endpoint, `stream_was_cancelled` remained
False because `RunCancelledException` was raised in the consumer code (wrapper),
which closes the generator from outside. This causes Python to skip the
generator's except blocks and jump directly to finally with the wrong flag value.

**Solution:**
1. Shared `asyncio.Event` registry for cross-layer cancellation signaling
2. `cancellation_aware_stream_wrapper` sets the event when cancellation detected
3. Wrapper uses `.athrow()` to inject exception INTO generator (not consumer-side raise)
4. All streaming interfaces check event in `finally` block to set flag correctly
5. `streaming_service.py` handles `RunCancelledException` gracefully, yields [DONE]

**Changes:**
- streaming_response.py: Event registry + .athrow() injection + graceful handling
- openai_streaming_interface.py: 3 classes check event in finally
- gemini_streaming_interface.py: Check event in finally
- anthropic_*.py: Catch RunCancelledException
- simple_llm_stream_adapter.py: Create & pass event to interfaces
- streaming_service.py: Handle RunCancelledException, yield [DONE], skip double-update
- routers/v1/{conversations,runs}.py: Pass event to wrapper
- integration_test_human_in_the_loop.py: New test for approval + cancellation

**Tests:**
- test_tool_call with cancellation (OpenAI models) ✅
- test_approve_with_cancellation (approval flow + concurrent cancel) ✅

**Known cosmetic warnings (pre-existing):**
- "Run already in terminal state" - agent loop tries to update after /cancel
- "Stream ended without terminal event" - background streaming timing race

👾 Generated with [Letta Code](https://letta.com)

Co-authored-by: Letta <noreply@letta.com>

letta/server/rest_api/routers/v1/conversations.py

@@ -289,11 +289,14 @@ async def retrieve_conversation_stream(
     )
 
     if settings.enable_cancellation_aware_streaming:
+        from letta.server.rest_api.streaming_response import cancellation_aware_stream_wrapper, get_cancellation_event_for_run
+
         stream = cancellation_aware_stream_wrapper(
             stream_generator=stream,
             run_manager=server.run_manager,
             run_id=run.id,
             actor=actor,
+            cancellation_event=get_cancellation_event_for_run(run.id),
         )
 
     if request and request.include_pings and settings.enable_keepalive: